Opiates have been the subject of intense research since the isolation of morphine in 1805, and thousands of compounds having opiate or opiate-like activity have been identified. Many opioid receptor-interactive compounds, including those used for producing analgesia (e.g., morphine) and those used for treating drug addiction (e.g., naltrexone and cyclazocine) in humans work by triggering μ opioid receptors in the central nervous system (CNS) and by crossing the blood-brain barrier. However, as there are μ opioid receptors elsewhere in the body, i.e., peripheral to the CNS and brain, sometimes these opiates can cause unwanted side effects. Often, these side effects target the gastrointestinal (GI) tract; for instance, prolonged morphine administration often causes constipation in patients. Thus, a drug that is able to treat symptoms of pain, for instance, but not stimulate the opioid receptors in the GI tract, would allow for pain relief without intestinal side effects.